Do I need a new magic Mouse for my MacOS Mojave, as although fully charged it is very slow and doesn't always work as required

For the current Magic Mouse, review the Mac hardware compatibility list to ensure it is right for the age of your Mac. If so, then have you adjusted any of the System Preferences > Mouse settings, or pointer control settings in the Accessibility panel?

This is not the solution you are looking for, but over the years there have been a lot of postings on this forum concerning problems with a wireless keyboard or mouse. I am not a fan of them. I use and recommend a wired keyboard and mouse. Never a problem with either of them on my computer or my wife's computer.